Various derivatives of hexaaryl biimidazole are well known in the prior art as radical polymerization photoinitiators. All such photoinitiators are insoluble in water.
For example, U.S. Pat. No. 6,524,770 B1, “Hexaaryl biimidazole compounds as photoinitiators, photo sensitive composition and method of manufacturing patterns using the compounds”, issued Feb. 25, 2003 to Hidaka et al. and incorporated herein by reference in its entirety discloses a hexaaryl biimidazole compound useful as a photoinitiator, represented by Formula (I) shown in FIG. 1, wherein each R group represents an alkyl group which may be the same or different, and each X group is independently a fluorine or hydrogen. The compound (I) is particularly useful as an initiator in a photosensitive composition containing a polyimide precursor, which is curable under patternwise low radiation exposure to give a patterned layer having heat and chemical resistance.
For further example, U.S. Pat. No. 7,901,501 B2, “Ink jet composition”, issued Mar. 8, 2011 to Kobayashi, discloses an inkjet ink composition comprising: a white pigment; a polymerizable compound; and a polymerization initiator. The white pigment includes at least one of inorganic hollow particles or inorganic-organic hybrid hollow particles. The polymerization initiator may be a hexaaryl biimidazole.
In the prior art, hexaaryl biimidazoles (HABI) typically are manufactured in multiple steps. The first step involves the condensation of a benzaldehyde and a benzil, which may be substituted, with ammonium acetate in refluxing acetic acid to form a triaryl imidazole monomer comprising first, second, and third phenyl rings as shown in FIG. 1. The HABI dimer is then formed through oxidative coupling of the corresponding monomer. Both the benzaldehyde and benzil can be substituted at the various ring positions to give the HABI dimer specific characteristics such as improved solubility or photospeed.
U.S. Pat. No. 3,784,557, issued Jan. 8, 1974 to Cescon (“Cescon”) and incorporated by reference herein in its entirety discloses 2,4,5-triphenylimidazolyl radicals and their dimers (2,2′,4,4′,5,5′ hexaphenyl biimidazoles) having on the phenyl groups from 1 to 10 substituents free from a hydrogen atom capable of reacting with methyl magnesium iodide, one such substituent being in the ortho position on the 2-phenyl group and having a sigma value below 0.7. The 2-phenyl group can contain up to four substituents, while the 4 and 5 phenyl groups can contain up to three substituents each. The radicals and the dimers are stable and form a photochromic system, finding utility as components in sun shields or shades. They are prepared by oxidizing the corresponding substituted 2,4,5-triaryl imidazole to form the hexaaryl biimidazole which is dark stable. The radical forms upon exposure of the biimidazole to a light source and is stable in the presence of the light radiation.
Cescon does not disclose synthesis of carboxy-substituted hexaaryl diimidazoles and in fact teaches away from substituent groups comprising hydroxyl, thio, carboxyl, amino, and alkylamino. Surprisingly, it has now been found that a carboxy-substituted triaryl imidazole can be a useful intermediate in forming a water-soluble HABI salt in accordance with the present invention.
Cescon does disclose to perform the dimerization in the presence of an ethanol solution of an alkali, specifically KOH, but because the carboxy substituent is not taught in the disclosure and because the solvent is not water, the present invention is not anticipated.
In the prior art, all known HABI compounds are essentially water-insoluble, requiring the use of organic and/or other non-polar solvents to form solutions thereof, which places limitations on useful formulations of HABI compounds.
What is needed in the art are HABI compounds that are inherently water-soluble, e.g., salts of carboxylated HABI compounds.